The antibody response of cats to the feline oncornavirus-associated cell membrane antigen (FOCMA) is inversely correlated with tumor development and progression. This is true both under laboratory conditions, when cats are inoculated with the feline sarcoma virus or the feline leukemia virus (FeLV), and under field conditions, when cats are exposed to FeLV. Cats that have early and/or highly elevated serum antibody titers following exposure to virus resist tumor development. Conversely, cats that fail to develop significant levels of FOCMA antibody following virus exposure have a very high risk for tumor development. This response represents an immunosurveillance mechanism that operates efficiently under natural conditions. The FOCMA antigen is present on both cultured and biopsied leukemia or lymphoma cells. Virus structural proteins p 15, p 30, and gp 70 are also present on the same cells if they are actively producing virus. Adsorption of rabbit or goat antisera to the specific virus fractions, using either whole disrupted FELV or the specific fraction, resulted in elimination of reactivity. Adsorption of the feline sera using identical procedures did not eliminate FOCMA activity. Simultaneous assays of cat sera for antibody to p 30 and gp 70 by radioimmunoprecipitation gave similar results. Discordance was seen in a significant proportion of the samples for antibody to FOCMA as compared to antibody to p 30 or gp 70. BIBLIOGRAPHIC REFERENCES: Essex, M. 1975. Tumors Induced by Oncornaviruses in Cats. Pathobiol. Ann. 5:169-196. Essex, M., Sliski, A., Hardy, W.D. Jr., and Cotter, S.M. 1976. The Immune Response to Leukemia Virus and Tumor Associated Antigens in Cats. Cancer Res. 36:376-381.